Glow-worm 24hxi cycling and banging - help please!

[Catmandoo2]

Hello everyone. This looks to be a great forum.

I'm desperate for help and hopeful of finding it here.

My CH and DHW system was originally served by a Baxi Bermuda, replaced in December 2005 by Glow-worm 24hxi Boiler. Open Vented 2-valve System, with indirect HW Cylinder, 11-rads (10 with stats) and a skirting heating installation, with stat.

System worked fine until recently when boiler appears to ‘cycle’ more than usual, taking much longer to bring rooms and DHW up to desired temperature, and frequently ‘banging’ (kettling?) before boiler shuts down at around 74-deg C, remaining off as boiler temperature slowly drops and then repeating the cycle.

Installation company suspected possible ‘sludge’ in system and have carried out ‘powerflush’ through all radiators and boiler but this hasn’t cured the problem.

Any ideas as to what could be causing the problem?

Note, it occurs when running either just the DHW system, the CH system, or both.

I believe my 24hxi boiler was an early version – is this a known fault with this model perhaps?

Don’t know where to go from here. Any thoughts and advice greatly appreciated.

 

[picasso]

Could be the pump is on its way out, probably worth changing even if its only to rule it out.

 

[Catmandoo2]

Thanks Picasso. Pump is a WILO Gold RS50 running at setting 3.

It was new with the boiler installation Dec 2005. It still looks good, sounds good, and seems to have no problem circulating throughout the system.

The two guys visiting from the installation company both thought it was fine.

 

[mickyg]

pump or blocked heat exchanger. Pumps likely too small for that boiler anyway. Hope its already on max speed

 

[Catmandoo2]

Thanks mickyg.

The pump is running on max, setting 3, and has always appeared adequate for the system since it was installed in Dec 2005.

Is there any easy way of testing the pump and hx?

 

[mickyg]

Thanks mickyg.

The pump is running on max, setting 3, and has always appeared adequate for the system since it was installed in Dec 2005.

Is there any easy way of testing the pump and hx?

if the pump "appears to be running correctly then I would remove the pump head and inspect the impellor, Wilo pumps have a habit of blocking up on the impellor.
did you pay for the powerflush that didnt help?

 

[Catmandoo2]

Thanks mickyg.

"If the pump "appears to be running correctly then I would remove the pump head and inspect the impellor, Wilo pumps have a habit of blocking up on the impellor. "

I'll take another look at it then.

Is the theory behind the banging and cycling I'm experiencing due to insufficient flow through the boiler - causing the water in the boiler to heat-up too quickly and resulting in the boiler shutting down until the water in the boiler cools down again?

Trying it today, with just the DHW system selected, the boiler quickly reached the set point of 70deg (with just a little grumbling rather than banging) and then shut down. It fired up again soon after as the temperature slowly dropped to around 40deg, this time staying on for several minutes - with burner modulating around 69deg before shutting down. This was followed a few minutes later by a short blip - quickly reaching the set point and closing down, presumably having brought the DHW cylinder content up to set point. In all this there was no banging and just a hint of boiling.

With the current hot ambient temperatures I'm not really able to put the CH system to a worthwhile test, but I suspect it will still run on the edge of 'kettling' and continue to cycle more than it should do.

"did you pay for the powerflush that didn't help?"

Not yet, but they are still to come back to me. I was disappointed to find that they did not carry out a de-scaling of the boiler. I would have thought that was a priority - more so than the system powerflush.

 

[Catmandoo2]

Following mickyg's recommendation, I opened-up the WILO pump and found the impeller quite heavily coated with red rust. In taking it out to clean it, I found that almost all the impeller internal pathways were filled with this rust - it is amazing it produced any flow at all when running.

Should it really have still been like that following the installer's "power-flush" of the system?

Should there be red rust in the system (rather than black sludge)? Does it indicate insufficient inhibitor perhaps? How often need inhibitors be added to a system like mine? Is there a simple test to confirm adequate levels in the system?

I have now thoroughly scoured out all the rust from the impeller, reassembled the pump, and tested the DHW heating system. Certainly the flow seems to have improved. On start-up the boiler quickly heated up a slug of water and shut down again. This slug of hot water was then circulated many times around the DHW system before the boiler eventually fired again and this time remained on and modulating until the output temperature reached set-point - performing much as described in my last post above.

Is this how it should work?

Is my understanding of the theory (as described in my last post above) correct?

I have yet to try out the full heating system again, but hope to do so soon as the outside ambient temperature is now dropping quite fast.

Apologies for so many questions, but desperately trying to resolve these problems with the help of the expertise in here.

 

[mickyg]

when you say red, do you mean more like orange? This is general corrosion often from an old cast iron heat exchanger boiler of past. The pump was obviously blocked before they flushed it so with any luck they've now cleared out whats in the system. I would be tempted to replace the pump with a more powerful 6m head one or downrate the boiler. In actual fact downrating the boiler is probably a good idea regardless, I'd be surprised if you actually need 24kw anyway, and trying to shove that into a standard cylinder will generally cause cycling of the boiler.
If you have the manal you should be able to get into the boiler "d" codes and alter d 0 to say 15kw for now and see how you go. how big is the property?

 

[Catmandoo2]

Thanks mickyg

when you say red, do you mean more like orange?...

Well, yes, rust-colour rather than black sludge.

...This is general corrosion often from an old cast iron heat exchanger boiler of past.

Hmm, that's disturbing. The installation company did the full conversion in December 2005, throwing out the old Baxi Bermuda Boiler and installing the new 24hxi based system complete with new HW storage cylinder and pump and a new power shower. All 10 rads and skirting heating and piping between them were existing, but all rad-stats replaced, and they carried out two power-flushes of the system on completion during recommissioning following the upgrade. There certainly should not have been any residual corrosion from the old boiler remaining in the system after that.

...The pump was obviously blocked before they flushed it so with any luck they've now cleared out whats in the system.

I would hope so, but I'm not too confident about that.
They were asked in to resolve the cycling and banging problem, and I thought they would have checked and cleared the pump first, and also de-scaled the boiler hex, to eliminate them as possible problems, before even resorting to a full system power-flush. Would that not be realistic?

The whole system has worked fine from Dec 2005 until now.

...I would be tempted to replace the pump with a more powerful 6m head one or downrate the boiler. In actual fact downrating the boiler is probably a good idea regardless, I'd be surprised if you actually need 24kw anyway, and trying to shove that into a standard cylinder will generally cause cycling of the boiler.
If you have the manal you should be able to get into the boiler "d" codes and alter d 0 to say 15kw for now and see how you go. how big is the property?

The system serves 10-rads (3 of which are doubles) plus a skirting heated lounge, a power shower and DHW system to two bathrooms and a kitchen, all single storey. It copes well throughout winter, but can also supplemented by a gas fire in the living room if needed.

From that experience of use, I'd be reluctant to down-rate the boiler. Is it not capable of modulating its output to cope with light loads?

 

[Agile]

Red rust indicates a recently produced iron oxide. Possibly from pumping over in your case.

After a few months it turns to black magnetite.

You should check that there is no more pumping over and no leaks in your system causing constant topping up.

Tony

 

[Catmandoo2]

Thanks Tony.

Does "Pumping Over" mean primary water being forced back into the feed tank, for some reason, and causing an overflow?

I'm not aware of any overflow having occurred, nor of any leaks, nor of the system taking on water from the feed tank.

I have wondered about the inhibitor content ever since the system was upgraded in Dec 2005 though. The installer finished in a bit of a rush, having had to power-flush a second time to get all th3e rads working, and I'm not sure that it was left fully protected. Could that have resulted in rust formation?

Is there any way of checking the inhibitor content of a running system?

 

[mickyg]

the boilers oversized for the system. Set your hot water to come on 1hr before the heating in the winter and rate it down to around 15kw, it will cope with that load no problem. The problem with these boilers is they dont know that they are oversized until they've fired up and the return temp doesnt come back quick enough. Its just the nature of the way they work....Light at 70%, ramp to max rate until it knows any better to ramp down again. With small loads like on HW only it could potentially overshoot, shut down and cycle just as you have described.
What kind of hot water cylinder do you have ? new/old?

 

[Catmandoo2]

Thanks mickyg

Yes, that is how the boiler seems to operate. Once the return water increases in temperature the boiler appears to modulate fairly happily at a low level.

The hot water cylinder details:
RCM Group Superheat High Performance Multi-coiled, copper, insulated, of approx dimensions 760 x 480mm, fitted with immersion heater as standby. It feeds two bathrooms, kitchen and a power-shower.

I've done a quick check on my radiators. The total surface area is around 13.5 sq. ms. (Whoops! That's the overall dimensions of the rads, H x L. The total surface area of course will be double that - sorry for the error). Most piping is in the loft, with 6-drops down to floor level feeding the rads.

We rarely have all rads fully on, since they are all controlled by rad-stats many of which are turned right down in rooms not in use.[/i]

 

[mickyg]

i would definately downrate it and see how it performs, you can always put it back again, the lower it will work the better it will work, at least though it sounds like the coil rating of the cylinder is decent enough. According to the copperform brochure your cylinder will still reheat in 14minutes with 15kw input.